1. Field of the Invention
Embodiments of the present invention generally relate to methods and apparatus for fabricating optical devices, such as optical integrated circuits (ICs).
2. Description of the Related Art
Optical systems are emerging technologies that offer solutions to many previously unsolvable technological problems. Thus, optical systems are now gaining an ever increasing importance in the technology world of today. Generally, optical systems utilize pulses of light rather than electric current to carry out such functions as data transmission, data routing, or other forms of data communication or data processing. One important structure commonly utilized in optical systems is an optical waveguide.
Optical waveguides are used to confine and direct light between the various components of an optical system. For example, optical waveguides may be used to carry Dense Wavelength Division Multiplexed (DWDM) light, which is used to increase the number of wavelengths in a single waveguide to achieve a higher aggregate bandwidth. FIG. 1 is a cross-sectional view of an optical fiber waveguide. The general structure of an optical waveguide 100 comprises two principal components: a core 103 surrounded by one or more cladding layers 102, 106. The core 103 is the inner part of the fiber through which light is guided. It is surrounded completely by the cladding layers 102, 106, which generally have lower refractive indexes than the core 103 to allow a light ray 105 in the core 103 that strikes the core/cladding boundary at a glancing angle to be confined within the core 103 by total internal reflection. The confinement angle θc  represents an upper limit for the angle at which the light ray 105 can strike the boundary and be confined within the core 103.
Fabrication of planar optical components on silicon and silica substrates currently exists. The waveguide must be isolated from the silicon substrate to avoid interfering with the light wave traveling down the waveguide as shown with the cladding layers 102, 106 in FIG. 1. Light waves traveling in a waveguide comprise two orthogonally polarized modes. For waveguide applications, one polarization is horizontal to the substrate and the other polarization is orthogonal to the substrate. If the lower cladding is too thin, the two orthogonal modes see a different effective refractive index resulting in birefringence, a consequential dispersion phenomenon that would limit the width of the transmission window.
A conventional waveguide structure requires at least three deposition steps and one mask level. For example, the lower cladding layer must first be deposited to isolate the substrate from the waveguide structure. Next, a core layer is deposited and patterned with a mask layer to form the waveguide paths. An upper cladding layer is then deposited thereover. The upper cladding layer must be thick enough to prevent interference from external ambient light, i.e., light from the environment outside the device. In addition, each of these layers may, and currently do, require post deposition heat treatment to obtain the desired optical properties.
However, the use of traditional mask materials to pattern and etch the core material 103, such as photoresist and/or silicon nitride has resulted in difficulties in forming the patterned core materials 103. Further, photoresist material and silicon nitride have had difficulties in being removed from the core material without forming defects in the core material, such as malformed features and roughing the core material surfaces. Such defects may result in propagation loss or attenuation, one optical waveguide core characteristic that is critical to the performance of an optical system.
Attenuation refers to the loss of light energy as a pulse of light propagates down a waveguide channel. The two primary mechanisms of propagation loss are absorption and scattering. Absorption is caused by the interaction of the propagating light with impurities in or on the waveguide channel, such as insufficiently removed mask residues. For example, electrons in the impurities may absorb the light energy and undergo transitions or give up the absorbed energy by emitting light at other wavelengths or in the form of vibrational energy (i.e., heat or photons). The second primary mechanism, scattering, results from imperfections in the surfaces of the core materials that cause light to be redirected out of the fiber, thus leading to an additional loss of light energy.
Thus, there is a need for an improved method of manufacturing optical waveguides with minimal propagation loss.